The present invention relates to an optical information recording and playback method for projecting a light spot onto an optical recording medium by means of a laser beam to record and/or play back information and optical disc memory apparatus. More particularly, the present invention relates to a light spot positioning method for rapidly positioning a light spot at a desired track by use of a macro moving mechanism (or coarse actuator) and a micro moving mechanism (or fine actuator) and an optical disc memory apparatus employing such light spot positioning method.
Optical disc memory apparatus have been developed as information storage apparatus of the type which records and plays back information from a high recording density-rotary recording medium or of the construction capable of erasing the recording medium as occasion demands.
The optical disc or the rotary recording medium is formed with a large number of tracks arranged concentrically or spirally at a given pitch and each of the tracks is provided with a large number of sectors for indicating the limits of data. In order to record external information at any given position or to play back or erase the information at any given position, there is required an access operation (seek operation) of searching first for selected one of the tracks on the disk surface and then looking for selected one of the sectors in the selected track. In other words, there exists a need for a macro seek control for rapidly moving a spot of light to the vicinity of a desired track, a track following control for maintaining the light spot on the center line of the desired track and a micro seek control for correcting the deviation between the light spot and the desired track. Such access operations in the optical disc memory apparatus are disclosed in JP-A-58-91536, JP-A-58-169370 and U.S. Pat. No. 4,607,358 combining these applications.
In the conventional optical disc memory apparatus, the positioning of a light spot is controlled by means of a macro moving mechanism, e.g., a linear motor for moving an optical head and a micro moving mechanism, e.g., a voice coil for actuating a galvano-mirror or objective lens mounted on the optical head.
In other words, during the seek the record/playback optical head is first coarsely positioned with respect to a designated track on the optical disc by the macro moving mechanism such as a linear motor (macro seek control).
After the setting by this macro seek operation has been completed, a track following operation is performed temporarily through the cooperative operation of the macro moving mechanism and the micro moving mechanism (track following control).
Then, the track address of this location is read to determine the deviation between it and the desired track. To correct the deviation from the desired track, a track jump is repeated by the micro moving mechanism such as a galvano-mirror to move the light spot to the desired track (micro seek control). When the light spot is positioned at the desired track, the tracking control is again performed thereby completing the seek operation.
In order to reduce the macro seek time, the optical head must be driven at a high acceleration rate.
In the optical disc memory apparatus, a light spot converging lens actuator or the previously mentioned micro moving mechanism, optical components, etc., are mounted on the optical head and thus generally the rigidity of the head is small as compared with the magnetic disc apparatus. Thus during the macro seek, the bandwidth of the speed control servo system cannot be increased greatly. If the head is driven at a high acceleration without increasing the band of the speed control system, the speed deviation between its actual speed and the desired speed increases.
The desired speed curve of the macro seek is predetermined so that the speed is decelerated at the maximum acceleration for reducing the seek time and also on the desired track the relative speed of the light spot and the track is decelerated to such speed that permits pulling in the track. This track pulling-in enabling speed is limited by the magnitude of a track deviation signal detection range and the bandwidth of the light spot control loop and it is on the order of 3 mm/sec in the optical disc memory apparatus. On the other hand, where the acceleration for deceleration and the bandwidth of the velocity control servo system are respectively selected as 25 G, where G is a unit of acceleration, and 700 Hz as in the case of the magnetic disc apparatus, the velocity deviation becomes about 55 mm/sec. In other words, in the optical disc memory apparatus, the track cannot be pulled in. In the case of optical disc memory apparatus, the bandwidth of the velocity control system is decreased further and the velocity deviation is also increased, making it impossible to drive the optical head at such high acceleration as in the case of the magnetic disc apparatus. In addition to the velocity deviation, driving the optical head at the high acceleration gives rise to the problem of residual vibrations due to the previously mentioned low rigidity of the head. Even when the macro seek movement has been completed, the settling time for the vibrations of the head is so large that the seek time is not reduced much.
In the optical disc memory apparatus, generally the seek operation is performed in the two steps of the macro seek and the micro seek with the result that even if the speed of the macro seek is reduced to about the same as in the magnetic disc apparatus, the time required for the micro seek is correspondingly increased. The reason for requiring the micro seek resides in that the track pitch of the optical disc is as small as about 1.6 .mu.m. In other words, in view of such problems as the positioning accuracy of the macro moving mechanism, the accuracy of the head position detector and the large eccentricity of the optical disc, it is difficult to reach the desired track by the macro seek alone. Thus, the deviation from the desired track is corrected by the micro seek. On the contrary, in the magnetic disc memory apparatus the track pitch of the magnetic disc is as large as about 20 .mu.m and also the eccentricity of the disc is small as compared with the optical disc, thereby making it possible to reach the desired track by the macro seek alone. Thus, it is difficult to drive the optical disc memory apparatus at about the same acceleration as the magnetic disc memory apparatus and moreover the seek time cannot be reduced much even if the optical disc is driven at the high acceleration.
On the other hand, there is known the so-called cross-track seek as the seek system which does not perform the micro seek.
In this cross-track seek system, as disclosed in the previously mentioned JP-A-58-91536 or U.S. Pat. No. 4,607,358, the number of track passage pulses each generated upon traversing a track is counted and an optical head is positioned at the desired track by the use of a macro moving mechanism alone. However, where the cross-track seek is effected at a high speed, the header or the data signal written into the disc enters a tracking error signal and thus its band and the band of the track passage pulses overlaps each other, thus causing an erroneous counting of the number of tracks passed. As a result, the light spot cannot be positioned accurately at the desired track and a correction must be made by the micro seek, thus making it difficult for the cross-track seek system to reduce the seek time.
Due to these deficiencies, the optical disc memory apparatus has the disadvantage of requiring a long seek time as compared with the conventional memory apparatus, particularly the magnetic disc memory apparatus. The average seek time of a 12" optical disc memory apparatus is as slow as about 200 msec as compared with that of a 14" magnetic disc memory apparatus which is about 15 msec.